Satellites have the ability to provide line-of-sight communication paths to large geographical areas. Because of this, many different systems have been designed and are in various stages of development. These satellite based systems will provide communication and data services to a large number of mobile, portable and fixed subscriber equipment in many places around the world where users cannot presently be economically serviced by terrestrial based systems.
Modern satellite communication systems tend to use more than one satellite to fulfill their mission. Multiple satellites provide increased capacity, coverage and flexibility for the system. This multiplicity of satellites has led to the extensive use of inter-satellite links to provide system control and coordination. These relatively new factors in satellite systems provide opportunities to optimize the design of the satellites outside the traditional boundaries.
In the design of satellite communication systems, one of the concerns for system designers is the link performance. In general terms, the forward or reverse link performance is directly related to the power transmitted from the earth station or the satellite, the gains of the receiving antennas, path losses, and interference levels. Link gains which affect the satellite's configuration are provided by the on-board amplification and antennas. The transmitted power is a major design concern because it affects the mass and the primary power requirements of the satellite. Antenna size and configuration are also important considerations.
Other satellite considerations include transmitter duty cycle, modulation efficiency, power supply system conversion efficiency, heat dissipation, and temperature control. In general, any changes in these factors cause changes in the power output capability and usually correspond to an increase in the payload weight.
Satellites also have some form of thermal control to manage the temperature extremes. In a satellite, the temperature is controlled by balancing the amounts of radiated and absorbed energy. Internal inefficiencies in the components can generate heat, along with chemical reactions. External sources such as the sun also contribute to the heating of the satellite. Heat loss can be controlled by managing the emissivity of the satellite's surfaces.
The satellite system is a collection of multiple stage subsystems. In a most general view, the satellite system can be subdivided into a receive subsystem, a transmit subsystem, and a processing subsystem. What are needed are a method and apparatus which ensure more on-orbit capability by optimizing the hardware complexity, weight, power, and launch costs.
Interference is also a problem which must be dealt with by the satellite system designers. Interference can be due to other space-based transmitters or terrestrial-based transmitters. Interference can also be due to on-board sources. Therefore, what are also needed are a method and apparatus for minimizing interference problems.